1. Field of Invention
The present invention relates to an image file processing method, and more particularly to an image file decoding method.
2. Related Art
A digital camera uses a sensor to capture an image and converts the image into a digital signal to save the captured image into an digital image file format. Currently, one of the most popular image file format is Joint Photographic Experts Group (JPEG), and the JPEG format image file has been adopted by most digital cameras or other digital products.
After capturing an image, the user often browses the image files stored in the digital camera. However, no matter how high the resolution of the image stored in the digital camera is, the user can only view a reduced resolution image through a small-sized display screen. Therefore, when the user wants to clearly view a part of an image, the user needs to enlarge the image to view the details of the part of the image.
However, even if only a part of the image is expected to be enlarged and presented to the user, the direct processing method still needs to decode the entire image file to the full resolution and then display the part of the image designated by the user. This method requires a memory having an enough size for the full resolution image. Therefore, a large quantity of the memory size, the read and write bandwidth of the memory and the computation of decoding the image must be wasted. Along with the evolution of the day, the image resolution is improved quickly, which aggravates the overhead in decoding.
To reduce the required memory size, one choice is decoding a part of the image and displaying the part to the user. The relevant technique refers to the US Publication No. 7,480,418. Compared with the original file, in the prior art, the size of the image file is increased due to the overhead, which causes the decrease of the data compression ratio. Moreover, the processed image file is inapplicable to the current universal decoder (for example, JPEG decoder), so most of the flexibility is lost.